trigger to determine if any column changed

If you do not need to know if a column was updated, you can create a trigger that occurs instead of an update or after an update (but prior to the change committing). See this link: http://msdn.microsoft.com/en-us/library/ms189799.aspx
If you need to know a specific column changed you can use the UPDATE ( <column name> ) syntax. http://msdn.microsoft.com/en-us/library/ms187326.aspx

Sure.  You'd probably want to generate the code using sys.columns rather than writing it by hand, but the general structure would look like this:


CREATE TRIGGER <trigger_name>
ON dbo.<table_name>
AFTER UPDATE
AS
SET NOCOUNT ON;
UPDATE tn
SET edit_datetime = GETDATE()
FROM inserted i
INNER JOIN deleted d ON
    d.key_col = i.key_col
INNER JOIN dbo.<table_name> tn ON
    tn.key_col = i.key_col
WHERE
    ISNULL(i.data_col1, '~~') <> ISNULL(d.data_col1, '~~') OR
    ISNULL(i.data_col2, '~~') <> ISNULL(d.data_col2, '~~') OR
    ISNULL(i.data_col3, -999) <> ISNULL(d.data_col3, -999) --OR ...other columns

GO

Instead of rolling you own code, check out the SQL Server change tracking feature.  

http://msdn.microsoft.com/en-us/library/bb933875.aspx

This is the solution I went with.